I have the Nimbus button at the right-hand end of the address bar, and immediately after the update to 6.3.1 it has become twice the size and blurred - although it still works. I've noticed that if I drag it to the bookmarks bar, or to the main menu, it goes back to normal! I have an 'Addblock plus' Add-on button next to it and that is unchanged. Removing and replacing the Nimbus Add-on has no effect. I'm running Win 7 on a 64 bit laptop and 32 bit netbook - it's the same on both, and Firefox 34.
Customer support service by UserEcho